Hydrogen storage material and method of using the same

ABSTRACT

In one embodiment, a method of using hydrogen includes forming a crystalline solid mixture of a metal halide M 2 (H L ) y  with a metal borohydride M 1 (BH 4 ) x ; and forming an amorphous liquid mixture from the crystalline solid mixture.

TECHNICAL FIELD

The present invention relates to a hydrogen storage material involving an amorphous liquid mixture of a metal borohydride and a metal halide and method of using the same.

BACKGROUND

Certain metal hydrides and metal borohydrides have been used for hydrogen storage. However, these metal hydrides and metal borohydrides have limited use because of necessary energy expenditures involved in maintaining relatively high temperatures for hydrogen desorption. In addition, borohydrides are not typically able to be rehydrided after hydrogen release. There is a continuing need for hydrogen storage materials with industrially acceptable cost efficiency for operation and maintenance.

SUMMARY

In one embodiment, a method of using hydrogen includes forming a crystalline solid mixture of a metal halide M²(H_(L))_(y) with a metal borohydride M¹(BH₄)_(x); and forming an amorphous liquid mixture from the crystalline solid mixture.

The method may further include forming a hexagonal metal borohydride[h-M¹(BH₄)_(x)] from an orthorhombic metal borohydride [o-M¹(BH₄)_(x)] and combining the former with the metal halide to form the crystalline solid mixture.

The method may further include combining the amorphous liquid mixture with a metal hydride M³H_(z) to form a hydrogen storage material.

The method may further include heating the hydrogen storage material to form a dehydrogenated composition while hydrogen is released. The method may further include cooling the dehydrogenated composition to form a cooled dehydrogenated composition. The method may further include inputting hydrogen into the cooled dehydrogenated composition to form a recharged hydrogen storage composition.

In another embodiment, a hydrogen storage material includes a crystalline solid mixture of a metal borohydride M¹(BH₄)_(x) and a metal halide M²(H_(L))_(y); and an amorphous liquid mixture formed from at least a portion of the solid solution, wherein, upon an X-Ray Diffraction analysis, the crystalline solid mixture presenting one or more peaks and the amorphous liquid mixture presenting no measurable peaks.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A illustratively depicts kinetic desorption data to 1 bar H₂ for 6LiBH₄/CaH₂/xTiCl₃ nano-composites as a function of temperature;

FIG. 1B illustratively depicts DSC curves of the 6LiBH₄/CaH₂/xTiCl₃ nano-composites;

FIG. 1C illustratively depicts room-temperature (RT) PXRD (Powder X-Ray Diffraction) patterns and corresponding phase identification for the post-milled 6LiBH₄/CaH₂/xTiCl₃ nano-composites;

FIG. 1D illustratively depicts RT PXRD patterns for 6LiBH₄/CaH₂/xTiCl₃ nano-composites after desorption to 500° C.;

FIG. 2A illustratively depicts hydrogen absorption/desorption reversibility of 6LiBH₄/CaH₂ nano-composite;

FIG. 2B illustratively depicts hydrogen absorption/desorption reversibility of the 6LiBH₄/CaH₂/0.25TiCl₃ nano-composite;

FIG. 2C illustratively depicts PXRD patterns of the two nano-composites after recharging and cooled to room temperature in air;

FIG. 3A illustratively depicts In-situ PXRD patterns for 6LiBH₄/CaH₂/0.25TiCl₃ nano-composite from 40° C. to 300° C.;

FIG. 3B illustratively depicts PXRD patterns of quenched 6LiBH₄/CaH₂/0.25TiCl₃ nano-composites after desorbing for 10 hours at 300° C., 350° C., 400° C., 450° C., and 500° C., respectively; and

FIG. 4A-4B illustratively depict schematic microstructure and phase transformations of 6LiBH₄+CaH₂+0.25TiCl₃ nano-composite: (4A) after-milling; (4B) 120° C. to 280° C. (LiBH₄+LiCl solid solution); (4C) 280° C. to 385° C. (LiBH₄+LiCl molten solution); and (4D) after main desorption forming CaB₆ and LiH, and precipitation of LiCl.

DETAILED DESCRIPTION

As required, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention that may be embodied in various and alternative forms. The figures are not necessarily to scale; some features may be exaggerated or minimized to show details of particular components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for teaching one skilled in the art to variously employ the present invention.

Except where expressly indicated, all numerical quantities in this description indicating amounts of material or conditions of reaction and/or use are to be understood as modified by the word “about” in describing the broadest scope of the present invention.

The description of a group or class of materials as suitable for a given purpose in connection with one or more embodiments of the present invention implies that mixtures of any two or more of the members of the group or class are suitable. Description of constituents in chemical terms refers to the constituents at the time of addition to any combination specified in the description, and does not necessarily preclude chemical interactions among constituents of the mixture once mixed. The first definition of an acronym or other abbreviation applies to all subsequent uses herein of the same abbreviation and applies mutatis mutandis to normal grammatical variations of the initially defined abbreviation. Unless expressly stated to the contrary, measurement of a property is determined by the same technique as previously or later referenced for the same property.

Complex hydrides are considered promising candidates for solid-state hydrogen storage materials because of their high theoretical hydrogen storage capacity and widely tailorable hydrogen desorption temperatures through forming mixtures with other materials. Within this class of materials, metal borohydrides, such as LiBH₄, Mg(BH₄)₂ and Ca(BH₄)₂, have received special attention due to their high theoretical gravimetric and volumetric hydrogen densities. These metal borohydrides alone are limited for practical on-board fuel cell hydrogen storage application at least because they often require relatively high temperatures for hydrogen releases.

In one or more embodiments, the present invention is advantageous in providing a ternary system of a metal hydride, a metal borohydride and a metal halide for storing and releasing hydrogen. Without wanting to be limited to any particular theory, it is believed that addition of the metal halide into the metal borohydride helps to form an intermediary compound between the two, which, upon reaction with the metal hydride, facilitates hydrogen releases in a relatively lower temperature range.

In one embodiment, a method of using hydrogen includes forming a crystalline solid mixture of a metal halide M²(H_(L))_(y) with a metal borohydride M¹(BH₄)_(x); and forming an amorphous liquid mixture from at least a portion of the crystalline solid mixture. The method may further include forming a hexagonal metal borohydride[h-M¹(BH₄)_(x)] from an orthorhombic metal borohydride [o-M¹(BH₄)_(x)] and combining the former with the metal halide to form the solid solution.

The term “crystalline solid mixture” may alternatively be referred to as a solid solution. The term “amorphous liquid mixture” may alternatively be referred to as a molten liquid solution.

In the solid solution, the metal borohydride M¹(BH₄)_(x) and the metal halide M²(H_(L))_(y) are mixed such that the solid solution is substantially homogenous. The term “substantially” may refer to the extent of the homogeneity, in that less than 15 percent, 10 percent, 5 percent, or 1 percent by weight of the total weight of the solid solution consists of either the metal borohydride M¹(BH₄)_(x) or the metal halide M²(H_(L))_(y) alone.

In the liquid molten solution, the metal borohydride M¹(BH₄)_(x) and the metal halide M²(H_(L))_(y) are mixed such that the liquid molten solution is substantially homogenous. The term “substantially” may refer to the extent of the homogeneity, in that less than 15 percent, 10 percent, 5 percent, or 1 percent by weight of the total weight of the liquid molten solution consists of either the metal borohydride M¹(BH₄)_(x) or the metal halide M²(H_(L))_(y) alone.

For the metal borohydride M¹(BH₄)_(x) and metal halide M²(H_(L))_(y), M¹ and M² are each independently an alkali metal or an alkaline earth metal, with x and y to charge balance. In certain instances, the values of x and y are 1≦x≦2 and/or 1≦y≦2.

The term “hexagonal” may refer to a crystal structure including one or more of the following features: the lattice parameters on the base plane are substantially equal to each other, the angle between the lattice parameters is about 120°, the vertical axis is substantially perpendicular to the basal plane, and the lattice parameter for the vertical axis different from one or both of the lattice parameters on the base plane.

The term “orthorhombic” may refer to a crystal structure including one or more of the following features: the lattice parameters on the base plane are substantially not equal to each other, the vertical axis is substantially perpendicular to the basal plane, and the lattice parameter for the vertical axis is different from both of the lattice parameters on the base plane.

The hexagonal metal borohydride[h-M¹(BH₄)_(x)] may include one or more lattice cavities, and at least a portion of the metal halide M²(H_(L))_(y) may be positioned within the one or more lattice cavities of the hexagonal metal borohydride[h-M¹(BH₄)_(x)]. When present at an elevated temperature, the metal halide M²(H_(L))_(y) and the hexagonal metal borohydride[h-M¹(BH₄)_(x)] together form a molten solution having a viscosity value greater than a viscosity value of the hexagonal[h-M¹(BH₄)_(x)] compared at the elevated temperature.

To the molten solution a metal hydride M³H_(z) may be added. M³ is an alkali metal or an alkaline earth metal, with z to charge balance. In certain instances, the value of z is 1≦z≦2. Optionally the metal borohydride M¹(BH₄)_(x), the metal halide M²(H_(L))_(y) and the metal hydride M³H_(z) may be all packaged in a container prior to one or more of the heating steps as described herein.

The orthorhombic metal borohydride [o-M¹(BH₄)_(x)] may be heated to form the hexagonal metal borohydride[h-M¹(BH₄)_(x)]. Prior to heating, the orthorhombic metal borohydride [o-M¹(BH₄)_(x)] may be kept at room temperature or 25 degrees Celsius. The heating may be carried out at a constant heating rate, at a variable rate, or both. Non-limiting examples of the heating rate include a heating rate of from 0.1 to 200° C. per minute. For the purpose of heating, any suitable heating devices may be used. Non-limiting examples of the heating devices include resistive heating band, catalytic hydrogen burner, flash light heating, and exhaust heating from the coolant of fuel cell or internal combustion engines. Any subsequent steps may be initiated when 90 weight percent or more of the orthorhombic metal borohydride [o-M¹(BH₄)_(x)] has been transformed into hexagonal metal borohydride [h-M¹(BH₄)_(x)]. In certain instances, the heating may be carried out at a first temperature of from no less than 15° C., 25° C., 35° C., 45° C., 55° C., 65° C., 75° C., 85° C., or 95° C., to no more than 180° C., 170° C., 160° C., 150° C., 140° C., 130° C., 120° C., or 110° C.

The solid solution of M²(H_(L))_(y) and M¹(BH₄)_(x), and particularly [h-M¹(BH₄)_(x)], may be formed at a second temperature greater than the first temperature. The liquid molten solution may be formed from the solid solution at a third temperature greater than the second temperature. The mixture may be formed as a molten solution of M²(H_(L))_(y) and [h-M¹(BH₄)_(x)] with a viscosity value greater than a viscosity value of M¹(BH₄)_(x) compared at a given temperature. The metal hydride M³H_(z) may be added to the mixture of M²(H_(L))_(y) and [h-M¹(BH₄)_(x)] after the formation of the mixture of M²(H_(L))_(y) and [h-M¹(BH₄)_(x)]. The second temperature may be of from no less than 90° C., 100° C., 110° C., 120° C., 130° C., 140° C., 150° C., or 160° C., to no more than 220° C., 210° C., 200° C., 190° C., 180° C., 170° C., 160° C., or 150° C. The third temperature may be of from no less than 190° C., 200° C., 210° C., 220° C., 230° C., 240° C., 250° C., or 260° C., to no more than 360° C., 350° C., 340° C., 330° C., 320° C., 310° C., 300° C., or 290° C.

The term “liquid molten solution” may refer to a homogeneous liquid formed when the solid solution melts. In this connection, there is no measurable compositional separation among the individual components contained within the molten solution. Because the liquid molten solution is a liquid and not a solid, the liquid molten solution presents no measurable peaks upon an X-ray diffraction analysis.

The term “viscosity” may refer to the easiness of movement of the molten solution. The more viscous the molten solution, the less likely the molten solution will segregate, facilitating the reaction between the metal hydride with the liquid molten solution.

The method may further include heating the hydrogen storage composition to form a dehydrogenated composition and to release hydrogen. One non-limiting benefit of the method is that now hydrogen may be released at a temperature such as 360° C. to 380° C., compared to a relatively higher temperature of from 390° C. to 430° C. The dehydrogenated composition may be cooled to form a cooled dehydrogenated composition. Fresh hydrogen may be inputted into the cooled dehydrogenated composition to form a recharged hydrogen storage composition.

Non-limiting examples of the metal borohydride M¹(BH₄)_(x) include lithium borohydride, sodium borohydride, potassium borohydride, calcium borohydride, strontium borohydride, barium borohydride, aluminum borohydride, and combinations thereof.

The halogen ion H_(L) of the metal halide M²(H_(L))_(y) may be a chloride, a bromide or an iodide. Non-limiting examples of the metal chloride M²(Cl)_(y) include magnesium chloride, calcium chloride, strontium chloride, barium chloride, zirconium chloride, titanium chloride and combinations thereof. Non-limiting examples of the metal bromide M²(Br)_(y) include magnesium bromide, calcium bromide, strontium bromide, barium bromide, zirconium bromide, titanium bromide and combinations thereof. Non-limiting examples of the metal iodide M²(I)_(y) include magnesium iodide, calcium iodide, strontium iodide, barium iodide, zirconium iodide, titanium iodide and combinations thereof.

Non-limiting example of the metal hydride M³H_(z) include lithium hydride, sodium hydride, magnesium hydride, calcium hydride, titanium hydride, and zirconium hydride, and combinations thereof.

Reversible incorporation of the metal borohydride M¹(BH₄)_(x) into and its precipitation out from the molten solution of [M¹(BH₄)_(x)].[M²(H_(L))_(y)] is believed to result in much improved reversibility. In this connection, the more viscous molten solution will prevent excessive clustering of the molten metal borohydride and the coalescence of metal hydride, preserving the nano-sized phase arrangement in the composites. During recharging, the reincorporation of metal halide into metal borohydrides as they form increases the viscosity of the molten solution, preserving the nano-sized solid-liquid phase arrangement, resulting in good hydrogen reaction reversibility.

Without wanting to be limited to any particular theory, it is believed that adding metal halides into metal borohydrides and metal hydrides nano-composites may improve the hydrogen storage property of the nano-composites in one or more of the following ways: via improving reaction thermodynamics, thus possibly decreasing the hydrogen desorption temperature; improving hydrogen reaction kinetics of the ternary system, and improving hydrogen reaction reversibility.

The metal borohydride M¹(BH₄)_(x), the metal halide M²(H_(L))_(y) and the metal hydride M³H_(z) may be combined using a ball mixing or milling procedure. Following the ball milling procedure, the mixed material may be subjected to a temperature treatment at a temperature of 180 to 300° C. One or more of the resulting products may have a lower hydrogen desorption temperature and faster desorption kinetics compared to the initial metal borohydride or metal hydride materials. Additionally, the third material of the process may be reversibly hydrogenated after release of an initial hydrogen composition.

As will be detailed in the Example section, a non-limiting ternary system may involve the following reaction(s). 6LiBH₄+0.75 LiCl+CaH₂

6LiBH₄.0.75LiCl+CaH₂

6LiBH₄.0.75LiCl+CaH₂

CaB₆+0.75LiCl+10H₂+6LiH. In this system, the three compounds are provided in a molar ratio of 6:0.75:1. These ratios may be particularly helpful to facilitate the formation of first a solid solution and later a liquid molten solution. When heated to certain temperatures such as 110° C. to 130° C., LiBH₄ and LiCl together form a solid solution 6LiBH₄.0.75LiCl. The presence of the liquid molten solution in comparison to the solid solution may be determined via X-ray Diffraction (XRD). The solid solution 6LiBH₄.0.75LiCl remains stable for a temperature range of up to 260° C. to 300° C., beyond which, the solid solution starts to melt to form a molten solution. Without wanting to be limited to any particular theory, it is believed that the solid solution and the liquid molten solution are each directed to a different state of the same chemical composition. In the state of the liquid molten solution, the 6LiBH₄.0.75LiCl molecule moves relatively easily and has relatively better contact with the other reactants.

Up till this point, CaH₂ does not react or is not a reactant yet, at least because CaH₂ is too stable to react. CaH₂ may be added in the beginning of the reactions or at a later time. CaH₂ may be added in the beginning along with all other ingredients to simply the process. When further heated to about 330° C. to 370° C., hydrogen releasing reaction occurs between the molten solution of LiBH₄.LiCl and CaH₂, releasing H₂ and forming CaB₆. In this process, as the molten LiBH₄.LiCl solution reacts with CaH₂, LiCl precipitates out. Therefore, the reactants are molten LiBH₄.LiCl and CaH₂ instead of molten LiBH₄ and CaH₂. This reaction pathway is believed to be coupled with favorable reaction thermodynamics and kinetics, and with lowered hydrogen desorption temperatures in particular. The reaction thermodynamics makes this reaction reversible and the reversibility may be examined by testing the charging/discharging repeatability.

The metal halide may be added to the metal hydride and/or the metal borohyride or may be formed as a reaction product using the metal borohydride as a reactant. For instance, LiCl may be formed through a replacement reaction between TiCl₃ and LiBH₄.

Example

Lithium borohydride (LiBH₄) (95% purity, Sigma-Aldrich), calcium hydride (CaH₂) (98% purity, Alfa-Aesar) and titanium chloride (TiCl₃) (95% purity, Sigma-Aldrich) are used as received. All sample handling is performed in aMBraunLabmaster 130 glovebox maintained under an argon atmosphere. Mechanical milling is carried out using a Spex 8000 high energy mixer/mill for samples loaded into a milling vial containing two stainless steel balls weighing 8.4 grams each. The mixture is ball milled for 5 hours. For the 6LiBH₄+CaH₂+xTiCl₃ system, its molar ratio is 6:1:x (x=0, 0.05, 0.1, 0.15, 0.2, 0.25 and 0.3).

Variable temperature hydrogen desorption kinetics are characterized using a water displacement desorption (WDD) apparatus where the desorbed gas amount is directly monitored as a function of temperature. For each experiment, a certain amount of sample is loaded into a stainless steel autoclave in the glove box. The sealed autoclave is mounted onto a three-port manifold connected to hydrogen purge gas as well as an outlet tube which passes through the bottom of a water-filled graduated burette. The manifold and sample are purged with hydrogen prior to each experiment. Each sample is heated at a rate (5° C./min) from room temperature to the final set point (up to 500° C.), and the desorbed hydrogen volume is monitored based on the amount of water displaced. The amount of desorbed hydrogen is corrected for the reduced headspace pressure and thermal expansion of 1 bar hydrogen gas upon sample heating. Dehydrogenation is performed in the WDD at selected temperature and hydrogen pressure, and reversibility is characterized through desorption using WDD.

Phase transformation and chemical reactions occurring when the samples are heated at a controlled heating rate are investigated using differential scanning calorimetry (DSC). Samples are placed inside aluminum crucibles and sealed with pierced lids made of the same material in the glove box. The data are collected under flowing helium (20 ml/min) within a temperature range of 20° C. to 500° C. using a heating rate of 5° C./min.

Phase identification and purity detection are characterized by Powder X-ray Diffraction (PXRD). PXRD data are collected on a SCINTAG (XDS2) powder diffractometer operated at 45 kV and 40 mA with step increments of 0.02° measured during 0.5 s using Cu Kα radiation (λ=1.5418 A^(O)). All samples are loaded in the glovebox with mineral oil to keep from air and maintained under a N₂ atmosphere during data collection. High-temperature X-ray diffraction data are collected using a Bueler HDK 2.4 furnace chamber attached to a Scintag X1 diffractometer, an Intel CPS 120 position sensitive detector and collimated Cu Kα radiation. Data are collected under an atmosphere of flowing purified nitrogen (200 sccm) while the temperature is ramped at a constant rate of 2° C./min from 4° C. to 300° C. with an interval of 40° C. Once the temperature reaches each set point, it holds at that temperature for 15 minutes to collect data. The phase identification above 300° C. is obtained using PXRD after desorbing the nano-composite at the selected temperatures until no further hydrogen is released and then the sample is cooled to room temperature rapidly by quenching into water.

In this experiment, samples of ternary compositions 6LiBH₄/CaH₂/xTiCl₃ with variable TiCl₃ concentrations (x=0, 0.05, 0.1, 0.15, 0.2, 0.25 and 0.3) are used. As detailed herein, LiCl is produced during ball milling of 6LiBH₄/CaH₂/xTiCl₃ and as temperature increases, o-LiBH₄ transforms into h-LiBH₄, into which LiCl incorporates, forming solid solution of LiBH₄.LiCl, which melts at or above 280° C. Molten LiBH₄.LiCl is more viscous than molten LiBH₄, preventing the clustering of LiBH₄ and the accompanied agglomeration of CaH₂, and thus preserving the nano-sized phase arrangement formed during ball milling. At or above 350° C., the molten solution LiBH₄.LiCl reacts with CaH₂, precipitating LiCl out of the molten solution LiBH₄.LiCl. The main hydrogen desorption reaction then proceeds between the molten solution LiBH₄.LiCl and CaH₂ and not between LiBH₄ and CaH₂. This alters the hydrogen reaction thermodynamics and lowers the hydrogen desorption temperature. In addition, the solid-liquid nano-sized phase arrangement in the nano-composites improves the hydrogen reaction kinetics. The reversible incorporation/precipitation of LiCl at the hydrogen reaction temperature and during temperature cycling (cooling down slowly and heating up through the phase transformation temperature of LiBH₄ at about 110° C.) makes the 6LiBH₄/CaH₂/0.25TiCl₃ nano-composite a fully reversible hydrogen storage material.

Samples 6LiBH₄/CaH₂/xTiCl₃ are ball milled for five hours to produce nano-composites. There are composites on the nano-meter size scale, wherein components of different phases have close contact with each other. The implications will be improved reaction kinetics due to good contacts between the phases. In some cases, the thermodynamic properties may be changed too due to the surface energy contributions. FIG. 1A shows the kinetic desorption data of each nano-composite of 6LiBH₄/CaH₂/xTiCl₃ system with a ratio 6:1:x (x=0.05, 0.1, 0.15, 0.2, 0.25 and 0.3). From FIG. 1A, the samples show at least two desorption steps: One smaller desorption step below 400° C. and the second main desorption step between 400° C. and 500° C. Adding TiCl₃ decreases the temperature of hydrogen desorption, and increases the amount of hydrogen released at the same temperature. The samples with 0.1, 0.15 and 0.2 TiCl₃ show similar dehydrogenation kinetics. The sample with 0.25 TiCl₃ illicits relatively greater dehydrogenation kinetics.

Phase transformation and hydrogen desorption through chemical reactions of the post-milled samples of 6LiBH₄/CaH₂/xTiCl₃ are also analyzed by differential scanning calorimetry (DSC) as shown in FIG. 1B. In each curve, three distinct endothermic peaks can be observed. The 1^(st) peak, observed around 110° C., is believed to correspond to the polymorphic phase transition of LiBH₄ from orthorhombic (o-LiBH₄) to hexagonal (h-LiBH₄) structure. The 2^(nd) peak, observed around 280° C., is believed to correspond to the melting of LiBH₄. The 3^(rd) peak corresponds to the main hydrogen desorption of the nano-composite between 400° C. and 500° C., which is consistent with the temperature range from the kinetic desorption data shown in FIG. 1A. Forming the molten solution of LiBH₄.LiCl does not necessarily lower the phase transition temperature (1^(st) peak) and the melting temperature (2^(nd) peak) of LiBH₄. However, for the 3^(rd) peak, which corresponds to the main hydrogen desorption, adding TiCl₃ significantly lowers the reaction temperature, especially at higher concentration of TiCl₃, which confirms that adding TiCl₃ improves the hydrogen desorption thermodynamics of the nano-composites.

Powder X-Ray Diffraction (PXRD) is used to identify the phases of the post-milled and desorbed nano-composites. FIG. 1C shows the PXRD patterns and corresponding phase identification for the post-milled 6LiBH₄/CaH₂/xTiCl₃ nano-composites. From FIG. 1C, it can be seen that the post-milled nano-composite of 6LiBH₄/CaH₂ (without adding TiCl₃) is a physical mixture of LiBH₄ and CaH₂. When TiCl₃ is added, peaks of LiCl begin to appear but no peaks of TiCl₃ are observed and the peaks of LiCl become stronger with increasing amounts of TiCl₃, accompanied by a loss of LiBH₄ peak intensity, while CaH₂ stays intact during ball milling. Taken together, these results suggest that LiBH₄ and TiCl₃ undergo a replacement reaction where LiCl is produced.

FIG. 1D shows the PXRD patterns for the 6LiBH₄/CaH₂/xTiCl₃ nano-composites after desorbing at 500° C. for 10 hours to 1 bar H₂ atmosphere. As shown in FIG. 1D, for low concentrations of TiCl₃ (x=0, 0.05) un-reacted LiBH₄ remains, at higher concentrations, the peaks of LiBH₄ and CaH₂ disappear while peaks of LiCl, LiH and CaB₆ begin to emerge and grow stronger. This suggests that the post-milled nano-composite samples may undergo one or more reactions by which LiH and CaB₆ are formed. Reaction shown below represents a non-limiting example: 3xLiCl+xTi(BH₄)₃+(6−3x)LiBH₄+CaH₂

(6−3x)LiH+CaB₆+(10+0.5x)H₂+3xLiCl+xTiH₂

The hydrogen absorption/desorption reversibility is also studied using nano-composites of 6LiBH₄/CaH₂ without 0.25 TiCl₃. FIG. 2A shows that for the first three cycles, the nano-composite of 6LiBH₄/CaH₂ shows a normal reversibility upon charging and desorbing at 425° C. When the desorption temperatures are decreased to 385° C. and 365° C. respectively (4^(th) and 5^(th) cycles), the desorption kinetics and the amount of desorbed hydrogen decrease significantly. After the recharging and desorbing temperatures were raised to 400° C. (6^(th) cycle) and 425° C. (7^(th) cycle), the desorbed hydrogen capacity is only partially recovered. On the other hand, FIG. 2B shows that the as-milled 6LiBH₄/CaH₂/0.25TiCl₃ nano-composite exhibits good reversibility for the first three cycles when desorbing at 385° C. When lowering the charging and desorbing temperatures to 365° C. and 345° C., both the kinetics and desorption capacity are decreased, as shown in the 4^(th) and 5^(th) cycles, respectively. However, when the recharging and desorbing temperature is set back to 385° C., shown as the 6^(th) cycle, the desorbed hydrogen amount and desorption kinetics return almost to the initial level. Therefore, it can be seen that adding 0.25 TiCl₃ into the 6LiBH₄/CaH₂ nano-composite can also significantly improve its reversibility.

FIG. 2C shows the PXRD patterns of the recharged 6LiBH₄/CaH₂ and 6LiBH₄/CaH₂/0.25TiCl₃ nano-composites after the reversibility cycles of FIG. 2A to 2C. As shown in FIG. 2C, after recharging in 170 bar H₂ at 425° C. and 385° C. respectively, both 6LiBH₄/CaH₂ and 6LiBH₄/CaH₂/0.25TiCl₃ nano-composites returns to a mixture of primarily LiBH₄ and CaH₂, which confirms the reversibility of the reaction between LiH+CaB₆ and LiBH₄+CaH₂ as shown in reaction (2).

Phase changes of the 6LiBH₄/CaH₂/0.25TiCl₃ nano-composite samples can be examined via in-situ PXRD. As depicted in FIG. 3A, LiBH₄ transforms from o-LiBH₄ to h-LiBH₄ between 80° C. and 120° C., which is consistent with the first endothermic peak (110° C.) in FIG. 1B. Above 280° C., CaB₆ and LiH begin to appear accompanied by a weakening of LiBH₄ and CaH₂ peaks, consistent with reaction (2). More importantly, the LiCl peaks become weaker as temperatures are increased from room temperature and disappear when the temperature exceeds 120° C. (phase transition temperature from o-LiBH₄ to h-LiBH₄), which suggests that LiCl incorporates into h-LiBH₄ to form a solid solution of LiBH₄.LiCl.

As LiBH₄.LiCl solid solution melts, the main hydrogen releasing reaction starts to take place. Hence, for the temperature from 300° C. to 500° C., five nano-composites are desorbed each at a specific temperature for 10 hours to 1 bar H₂ pressure (300, 350, 400, 450 and 500° C.), respectively, and then quenched to room temperature. Powder XRD patterns of those five quenched post-desorbed samples are used to identify phases after higher temperature hydrogen desorption, as shown in FIG. 3B. LiBH₄ and CaH₂ are observed in samples quenched at 300° C. to 350° C., although their peak intensities keep decreasing. Differences seen between FIGS. 3A and 3B at 300° C. may be attributable to the recrystallization of the molten LiBH₄. It is noteworthy that after desorption at 350° C. for 10 hours to 1 bar H₂, the quenched sample does not show LiCl peaks, suggesting that the molten solution of LiBH₄.LiCl at 350° C. is frozen into a solid solution. Above 400° C., peaks of CaB₆ and LiH become stronger along with the disappearance of LiBH₄ and CaH₂, which is consistent with reaction (2). Contrary to the case at 350° C., peaks of LiCl emerge again from 400° C. to 500° C. while LiBH₄ disappears due to the hydrogen reaction (2). Hence, during the hydrogen desorption process, LiCl in 6LiBH₄/CaH₂/0.25TiCl₃ nano-composite first forms solid solution (LiBH₄.LiCl) with LiBH₄, which becomes molten solution (LiBH₄.LiCl) as temperatures are increased to about 280° C., and then precipitates out after LiBH₄ reacts with CaH₂, forming CaB₆ and LiH.

The phase transformation of o-LiBH₄ to h-LiBH₄ and the dissolution of LiCl into h-LiBH₄ to form LiBH₄.LiCl first as solid solution and then as a molten solution can be observed via in-situ XRD. The LiBH₄.LiCl remains stable up to the onset of the main hydrogen reaction between the LiBH₄.LiCl (liquid) and CaH₂ (solid), forming CaB₆, LiH and releasing hydrogen. As LiBH₄ in the molten solution is being consumed, LiCl gradually precipitates out of the molten solution as a solid again. This process can be schematically illustrated in terms of microstructural transformations and the nano-sized phase arrangement, as can be observed from the XRD data of the quenched samples as shown in FIGS. 4A to 4D.

The reaction between LiBH₄ and TiCl₃ during ball milling forms LiCl through a replacement reaction, forming nanometer sized composite particles of LiBH₄+CaH₂+LiCl+TiH₂ (or Ti(BH₄)₃), as illustratively depicted in FIG. 4A. As this nano-composite is heated to about 120° C., LiBH₄ is transformed from orthorhombic into hexagonal structure, and LiCl subsequently incorporates into h-LiBH₄ to form a LiBH₄.LiCl solid (up to 280° C.) and molten solution (above 280° C.), as illustratively depicted in FIGS. 4B and 4C. The formation of the LiBH₄.LiCl solution changes the thermodynamics and the corresponding hydrogen desorption reaction and lowers the hydrogen desorption temperature from 425° C. (6LiBH₄+CaH₂ nano-composites) to 385° C.

The much improved hydrogen absorption/desorption reversibility of the 6LiBH₄+CaH₂+0.25TiCl₃ nano-composite compared to 6LiBH₄+CaH₂ nano-composite may result from the micro-structural change due to the incorporation of LiCl into LiBH₄, as illustratively depicted in FIGS. 4C and 4D. It is experimentally observed that the incorporation of LiCl into LiBH₄ increases the viscosity of the molten LiBH₄.LiCl compared to molten LiBH₄ at the same temperature. A more viscous LiBH₄.LiCl liquid helps to prevent the excessive clustering of the molten LiBH₄ and the coalescence of CaH₂, preserving the nano-sized phase arrangement in the 6LiBH₄+CaH₂+0.25TiCl₃ nano-composites which shortens the mass transfer distance during the hydrogen desorption reaction and preserves well dispersed CaB₆, LiH and LiCl nano-composites. It is also worth mentioning that the preformed TiH₂ may further prevent the grain growth of phases during the desorption process.

Similarly, upon recharging, the CaB₆ and LiH in the CaB₆+LiH+LiCl nano-composite formed during the hydrogen desorption will react to form LiBH₄ and CaH₂. Once LiBH₄ is formed, it will combine with the precipitated LiCl, forming a molten solution of LiBH₄.LiCl, which is more viscous than molten LiBH₄, preserving the nano-sized phase arrangement in the recharging process. This well-dispersed nano-sized solid-liquid phase arrangement in the 6LiBH₄+CaH₂+0.25TiCl₃ nano-composite helps to realize good reversibility.

In summary, systematic studies of the phase evolution on cycling 6LiBH₄/CaH₂/xTiCl₃ with x=0, 0.05, 0.1, 0.15, 0.2, 0.25 and 0.3 have been performed. Adding TiCl₃ and particularly 0.25 TiCl₃ produces reversible hydrogen absorption and desorption and a lower desorption temperature. LiCl is produced through replacement reaction between LiBH₄ and TiCl₃ during ball milling. This LiCl then forms solid solution with LiBH₄ at about 120° C. when o-LiBH₄ transforms into h-LiBH₄. The LiBH₄.LiCl solid solution persists up to about 280° C., where a molten solution of LiBH₄.LiCl results. This is attributable to the changes in reactant compositions and hence reaction thermodynamics, and to the favorable decrease in hydrogen desorption temperature. On the other hand, the incorporation of LiCl into LiBH₄ favorably changes the viscosity of molten LiBH₄.LiCl, preserving the nano-sized phase arrangement formed after milling, leading to fast kinetics. Furthermore, the precipitation of LiCl from viscous LiBH₄.LiCl molten solution with the consumption of LiBH₄ upon hydrogen desorption and its re-incorporation into LiBH₄ upon re-hydrogenation generates a well-dispersed liquid-solid nano-sized phase arrangement at the recharging temperature, leading to a fully reversible complex hydrogen storage system at the hydrogen reaction temperature range. The full reversibility of this system is also exemplified by the fact that when the recharged ternary nano-composite is cooled to room temperature slowly, LiCl precipitates from the molten LiBH₄.LiCl at temperatures below 120° C. As the temperature is increased in the next desorption, LiCl will re-incorporate into the h-LiBH₄ and the above cycle will continue.

While the best mode for carrying out the invention has been described in detail, those familiar with the art to which this invention relates will recognize various alternative designs and embodiments for practicing the invention as defined by the following claims. 

What is claimed is:
 1. A hydrogen storage system comprising: a hydrogen storage material including a metal borohydride M¹(BH₄)_(x), a metal halide M²(H_(L))_(y), and an amorphous liquid mixture of at least a portion of the metal borohydride M¹(BH₄)_(x) and at least a portion of the metal halide M²(H_(L))_(y), M¹ and M² each being independently an alkali metal or an alkaline earth metal, and x and y being to charge balance M¹ and M², respectively.
 2. The hydrogen storage system of claim 1, wherein the amorphous liquid mixture is substantially homogeneous.
 3. The hydrogen storage system of claim 1, further comprising a crystalline solid mixture of a portion of the metal borohydride M¹(BH₄)_(x) and a portion of the metal halide M²(H_(L))_(y).
 4. The hydrogen storage system of claim 3, wherein the crystalline solid mixture is substantially homogeneous.
 5. The hydrogen storage system of claim 3, a weight ratio of the amorphous liquid mixture to the crystalline solid mixture is greater than 1 at a first temperature and is smaller than 1 at a second temperature lower than the first temperature.
 6. The hydrogen storage system of claim 1, wherein the metal halide M²(H_(L))_(y) has a first weight at a first temperature and a second weight at a second temperature, the first temperature being at least 250 degrees Celsius, the second temperature is at least 100 degrees Celsius greater than the first temperature, and a weight difference between the first weight and the second weight being no greater than 10 weight percent.
 7. The hydrogen storage system of claim 1, further comprising a metal hydride M³H_(Z).
 8. The hydrogen storage system of claim 1, further comprising a storage vessel including the hydrogen storage material.
 9. The hydrogen storage system of claim 1, wherein x and y are each a value smaller than
 3. 10. A method of using hydrogen, comprising: forming a hydrogen storage material including an amorphous liquid mixture of a metal borohydride M¹(BH₄)_(x) and a metal halide M²(H_(L))_(y), M¹ and M² each being independently an alkali metal or an alkaline earth metal, and x and y being to charge balance M¹ and M², respectively.
 11. The method of claim 10, further comprising, prior to the step of forming the amorphous liquid mixture, forming a crystalline solid mixture of the metal borohydride M¹(BH₄)_(x) and the metal halide M²(H_(L))_(y).
 12. The method of claim 10, wherein the metal borohydride M¹(BH₄)_(x) includes a hexagonal metal borohydride [h-M¹(BH₄)_(x)] with one or more lattice cavities.
 13. The method of claim 12, wherein at least a portion of the metal halide M²(H_(L))_(y) is situated within the one or more lattice cavities of the hexagonal metal borohydride [h-M¹(BH₄)_(x)].
 14. The method of claim 12, further comprising forming the hexagonal metal borohydride [h-M¹(BH₄)_(x)] from an orthorhombic metal borohydride [o-M¹(BH₄)_(x)].
 15. The method of claim 10, further comprising releasing hydrogen from a reaction between the amorphous liquid mixture of the metal borohydride M¹(BH₄)_(x) and the metal halide M²(H_(L))_(y) and a metal hydride M³H_(z).
 16. The method of claim 15, wherein the reaction is carried out with elevated temperature.
 17. The method of claim 11, wherein the amorphous liquid mixture is formed at a higher temperature than the crystalline solid mixture.
 18. The method of claim 12, wherein the crystalline solid mixture is formed at a higher temperature than the hexagonal metal borohydride [h-M¹(BH₄)_(x)].
 19. The method of claim 15, wherein the metal hydride M³H_(z) is added after the amorphous liquid mixture is formed.
 20. A method of using hydrogen, comprising: forming a hexagonal metal borohydride [h-M¹(BH₄)_(x)] at a first temperature; forming a crystalline solid mixture of the hexagonal metal borohydride [h-M¹(BH₄)_(x)] and a metal halide M²(H_(L))_(y) at a second temperature higher than the first temperature; forming an amorphous liquid mixture of at least a portion of the hexagonal metal borohydride [h-M¹(BH₄)_(x)] and at least a portion of the metal halide M²(H_(L))_(y), at a third temperature higher than the second temperature; and releasing hydrogen via a reaction between the amorphous liquid mixture and a metal hydride M³H_(z) at a fourth temperature higher than the third temperature. 